Pyrolysis of a liquid hydrocarbon material is a well-known process that involves heating the material to a temperature that is high enough to cause thermal decomposition of larger molecules to form smaller molecules. Pyrolysis may be accomplished with a diluent, such as steam, to produce more favorable product distribution. A pyrolysis process produces a highly unsaturated and very unstable product, hereinafter called the effluent from the pyrolysis process, or simply the effluent.
The effluent is usually rich in olefins, diolefins, acetylenes and other highly unstable compounds, and there is a strong tendency for these materials to react to form high molecular weight products which may be identified collectively as coke or tar. Such products are not desirable and to avoid forming them it is essential to reduce the temperature of the effluent quickly to a stable temperature, that is, to a temperature that is so low that rapid reactions of unstable compounds with each other do not take place.
In at least one process of this type, the effluent is stabilized by indirect heat exchange in stages, while in another process, the effluent is first-precooled indirectly, and then stabilized by direct heat exchange with a liquid quench. In the latter process, the bulk of the heat absorbed by the quench liquid is removed in the later fractional distillation of the effluent and quench liquid, a significant portion of the heat removal being accomplished by separation of a bleed stream, heat exchange of the stream, and return of at least a portion of the bleed stream to the fractional distillation zone. This procedure, however, suffers from the deficiency that only low pressure steam may be generated, as well as requiring a large volume of bleed.
Great Britain Pat. No. 1,503,871, (corresponds to U.S. Pat. No. 4,150,716) describes a process in which the effluent is quenched at temperatures of up to 400.degree. C., the effluent and quench liquid contacting the walls of a shell and tube exchanger to generate high pressure steam. However, this procedure also does not utilize efficiently the high quality energy present in the pyrolysis effluent. Accordingly, a need has existed for a process that lessens coking problems while providing efficient heat energy recovery. The invention satisfies that need.